Hydraulic pressure of oil and power of electric motors are used in vehicle power steering. In particular, in use of hydraulic pressure, a hydraulic pressure pump pumps a fluid through power of an engine to form hydraulic pressure, and then, direction is switched and handling of the vehicle may become smooth by sending hydraulic pressure in a steered direction when a driver turns a steering wheel. In addition, motor driven power steering (MDPS) system uses a steering powder through motor power instead of hydraulic pressure. In the MDPS, an electric motor automatically performs control functions according to driving conditions, thereby having enhanced steering performance and steering feel, when compared with the hydraulic pressure-type power steering.
Such MDPS typically includes a motor for assisting a steering wheel, and properly assists operation force by electronically controlling output of a motor according to speed and steering torque. In addition, the MDPS includes a worm wheel coupled to a steering shaft extending therethrough and a worm shaft engaged with the worm wheel. In addition, the MDPS includes a tilt bearing such that engagement of the worm wheel and the worm shaft can be optimized. In addition, in the MDPS, a motor and a worm shaft are connected by a coupling as a separate connection member, thereby transferring motor power to a worm shaft.
However, the MDPS may have problems such as crackling dull noise upon release after steering of a steering wheel on waiting. In addition, when vehicles drive on a hard road or a bumpy road, great reaction force from the road is transferred to the steering shaft and, at this time, rattling noise may occur due to a gap between the worm wheel and the worm shaft. Such noise may frequently occur as an inner race constituting the tilt bearing moves in the axial direction of the worm shaft.
Accordingly, the coupling absorbs impact sound when the worm shaft moves in the axial direction. Conventionally, the couplings have been manufactured using urethane rubber. Although urethane rubber is resistant to repeated wear of a counterpart, when installed in a steering system, due to superior wear resistance thereof and has superior mechanical strength by power transfer and superior oil resistance and ozone resistance, the urethane rubber may be hydrolyzed in hot and humid environment, thereby decreasing functions of products.
In the related arts, Korean Patent Application Pub. No. 2003-0024046 discloses a certain power steering hose for high pressure including a certain inner-side rubber layer composed of a material that includes 50 to 80 parts by weight of carbon black, 10 to 30 parts by weight of zinc oxide, 10 to 30 parts by weight of an antioxidant, 1 to 10 parts by weight of a vulcanization accelerator and 0.1 to 2 parts by weight of a vulcanizing agent based on 100 parts by weight of hydrogenated nitrile-butadiene rubber. However, the product functions may be deteriorated due to hydrolysis.
In addition, Korean Patent Application Pub. No. 1998-079921 discloses a certain elastomer composition including hydrogenated acrylonitrile butadiene rubber, non-modified hydrogenated acrylonitrile butadiene rubber, a peroxide curing aid, an organic peroxide and carbon black. However, the elastomer composition may have a disadvantage such as poor mechanical strength against wear or impact.
Therefore, there is a need for a novel material which can prevent function decrease due to hydrolysis and breakage upon friction against coupling as a connection member or impact occurrence, even in hot and humid environments.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.